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Spinosaurids

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Scientists have long speculated that spinosaurids were fish eaters because they have long crocodile-like snouts, cone-like teeth, and fossilized stomach contents.[1] However, few have taken seriously the idea that they were aquatic or semi-aquatic because apart from their specialized skulls they have no obvious adaptations for a life in the water, and instead they resemble other theropods.[1] The authors however concluded that spinosaurids were semi-aquatic animals who spent as much time in the water as modern animals like crocodiles and hippopotamus.[1] They based this conclusion on a comparison of the oxygen isotopes present in the phosphates of spinosaurid fossils to those of the theropods, crocodilians, and turtles they lived alongside.[1] Spinosaurids having a semi-aquatic lifestyles and fish-rich diet may have eased competition for food and space with the carcharodontosaurids and tyrannosaurids that they shared their ecosystems with.[1]

Spinosaurids are known from Cretaceous strata in places like Africa, Europe, South America, and Asia.[2] The oldest known spinosaurids are known from Late Jurassic strata in Africa.[2] In 1984 Taquet was the first scientist to propose that spinosaurs were unusual for theropods, which he based on examination of fragmentary African fossils.[2] Baryonyx is a spinosaurid known from a relatively complete fossil skeleton found at the Wealden in southern England.[2] The long slender jaws of Baryonyx resemble those of longirostrine crocodiles, who are fisheaters.[2] The Baryonyx skeleton also preserved partially digested fish scales.[2] However, there is also "direct evidence" that spinosaurids exploited other food sources, including dinosaurs and pterosaurs.[2] Still, many scientists have concluded that the traits shared by crocodilians and spinosaurids are evidence for similar lifestyles between them.[2]

Apatites formed in the bodies of living things can vary in the amount of delta 18 oxygen present in their phosphates.[2] Factors responsible for such variations in homeothermic vertebrates include their physiology, ecology, and variation in the delta 18 oxygen contents of water they ingest from drinking and eating.[2]

Look up that "physiological adaptations specific to aquatic" stuff. Confusing sentence.[2]

In mammals and reptiles of both modern and fossil times, those who live in the same place, aquatic or semi-aquatic animals tend to have lower concentrations of delta oxygen 18 in their phosphates.[2]

The authors used 24 previously published delta oxygen 18 values of the phosphates of tooth enamel from spinosaurids, other theropods, crocodilians, and turtle shells.[3] They also measured 109 new values.[3] All of the measured fossils dated back to the Cretaceous within the age range from the Hauterivian to the Barremian to the early Cenomanian.[3] They chose fossils sites in fluvial to fluviodeltaic deposits from every continent with known spinosaurid fossils; Asia, Europe, South America, and Africa.[3] To avoid having oxygen isotopes resulting from differences in body size to confound their results the researchers compared spinosaur and terrestrial theropod teeth of similar sizes.[3] The researchers checked their measurements for statistical significance with a non-parametric Wilcoxon signed-rank test and a two way analysis of variance (ANOVA).[3]

The researchers found that the delta 18 oxygen in spinosaurid bone is 1.3% lower than in other kinds of theropods.[4] This amount is statistically significant. However, there was no statistically significant variation between spinosaurids, turtles, and crocodiles.[4] In location 12, the oxygen isotope values were comparable to theropods regarded as terrestrial.[4] In locations 6, 8, and 11 spinosaurid oxygen isotope levels spanned the whole range between crocodilians and turtles on the semi aquatic side to the terrestrial theropods.[4]

Microbe activity can alter the isotopic composition of the apatites in animals' bodies.[5] However, non-biological processes have little effect on the apatite in tooth enamel because their crystals are so large and densely arranged.[5] This is true even on timescales of millions of years.[5] Further, terrestrial theropods had different oxygen isotope values than semi-aquatic across both space and time.[6] If one site's fossils were altered after burial then all of the fossils at that site should have the same amount of isotopes.[7] The fact that there was still statistically significant variation between the terrestrial and semi-aquatic animals is evidence that at least some of the original variation of the living animals was retained by their fossils.[7] It's also not likely that the difference between spinosaurid and traditionally terrestrial theropods was attributable to the spinosaurids having a fish-based diet since the fossil evidence shows that they were opportunistic and fed on whatever they could catch.[7] This included fish, but also dinosaurs and pterosaurs. Spinosaurids both scavenged and ate live prey.[7] Their opportunistic diet would likely have been very similar to crocodilians, so if the spinosaurids were mostly terrestrial despite living on fish, their oxygen isotope quantities should be different from crocodilians.[7] However, the oxygen isotope values between spinosaurids and crocodilians were actually very similar.[7]

Oxygen istope abundance variation between spinosaurids and terrestrial theropods was due to the contents of their body water.[7] Semiaquatic animals lose less water from evaproation than terrestrial ones, which is one of the reasons that terrestrial animals have a higher relative amount of the heavier oxygen isotopes.[7] Semiaquatic animals like hippos and crocodiles also go through more drinking water than terrestrial animals.[7] Thus, spinosaurids's low levels of delta 18 oxygen was probably due to them having a semi-aquatic lifestyle.[7] The difference in isotope abundance between spinosaurids and terrestrial theropods was similar to the difference between those of hippos and animals like zebras, buffalos, elephants, and rhinos in two Kenyan national parks.[7] The difference in oxygen isotopes between hippos and other Kenyan herbivores parallels the same difference between spinosaurids and terrestrial theropods since in both cases the different groups had similar metabolisms, diets, and levels of thermoregulation, making the difference in habitat preferences the most likely source of their oxygen isotope variation. Hippos eat land plants like other African herbivores.[7]

Since spinosaurids show no obvious adaptations for life in the water, their time spent there may have served mainly to help control their body temperature.[7] Modern crocodilians and hippos do the same thing.[7] In all spinosaurid-bearing fossil sites other theropods of similar size are also found.[7] A semi-aquatic lifestyle may have eased competition for food and space between spinosaurids and these more terrestrial forms.[7] Although Spinosaurus had jaws with very obvious adaptations for fish-eating its Tunisian and Morroccan specimens showed no signs of a semi-aquatic oxygen isotope abundance.[7] Spinosaurus would have been in competition for food with other large theropods and large to giant crocodilians in lakes and rivers.[7] Some African spinosaurids were likely to have a "more opportunistic" diet as a consequence, exploiting food resources both on dry land and in the water.[7]

This study was the first to find "unambiguous" that some dinosaurs lived much of their life in the water rather than just visiting the water for short periods to hunt or fish.[7] The fact that dinosaurs are no longer thought to be purely terrestrial expands their known ecological diversity.[7]

East Berlin Formation swim tracks

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Lower Jurassic rocks near Rocky Hill, Connecticut preserve tracks left by a large theropod that Coombs interpreted as being made while the partially submerged dinosaur touched the bottom with its toes.[8]

Hadrosaurs were once widely believed to be amphibious, primarily on the basis of possible webbing between their fingers.[9] Other than that, the idea had very little support from scientific evidence.[9] Prior to Coombs's 1980 paper few scientists showed any interest in ascertaining theropod swimming abilities.[9] They typically assumed that theropods were such poor swimmers that herbivorous dinosaurs like sauropods and hadrosaurs could escape by entering the water.[9] Robert Bakker argued a contrarion opinion that theropods would have been competent swimmers who paddled by kicking with their hind legs, the way modern ostriches do.[9] State geologist Richard L. Krueger informed Coombs about the existence of strange dinosaur stracks in Connecticut State Dinosaur Park in Rocky Hill.[9] Coombs interpreted these tracks as support for Bakker's theory.[9]

John Ostrom reported that there were dinosaur tracks in the Rocky Hill area.[9] A temporary building was constructed over the tracksite to protect the footprints, but this was later blown over during a storm.[9] To protect the tracks they were buried until a permanent building could be built over the tracks.[9] Later, different set of prints were found at another site on what may be the same stratum in the Early Jurassic East Berlin Formation.[9] Coombs interpreted these tracks as having been made by a swimming theropod leaving marks on the bottom sediment with the tips of its toes while paddling through the water.[9]

These unusual tracks have an impression left by digit III that preserves a rounded claw mark and indendation made by the toe pad behind the claw.[9] The impression left by the toe pad lies forward of the claw prints left by digits II and IV.[9] Digit III is closer to digit IV than digit II and therefore isn't exactly centered.[9] The prints in both digits II and IV were left by the claw.[9] There are two impressions, the far impression is connected to the near impression by a thin channel cut into the sediment when the theropod drew its leg back.[9] The rear impression was made when the animal pushed off again after drawing back its leg.[9] Some of the claw marks have small piles of sediment at the rear of the digit II and digit IV impressions.[9] when the animal was paddling its third digit stayed still while the outer two slid a short distance backward.[10]

The track site contain 43 of the swim track described above. Most of them were left by large theropods, but some were left by smaller ones.[11] The quality of preservation ranges from "super[b]" to poor.[11] Some of these swim track ways were later imprinted upon by normal dinosaur walking tracks.[11] Some of the swim tracks penetrate deep into the sediment, others barely at all.[11] The longest swim trackway has eight tracks.[11] The average step length was 104.7 cm, although the step length between each step is different.[11] The right leg consistently had a longer step length than the left, with the right's average being 114.7 and the left being 97.3.[11] Coombs hypothesized that this discrepancy reflected a "galloping" pattern to its swimming.[11] A different trackway had five prints with an average step length of 130 cm, however its step length was symmetrical.[11]

The stratum's swim trackways start and end "abruptly", which Coombs felt was related to their origins as purported swim tracks.[11]

Coombs dismissed the possibility that his purported swim tracks were underprints on the grounds that underprints shouldn't have preserved the "delicate" scratch marks seen in the II and IV digits of the swim tracks.[11] Coombs didn't feel that these tracks could have been made in drying mud or in a thin layer of mud because such tracks should preserve the entire length of the digits.[11]

At Rocky Hill there are dark grey mudstones that were deposited along the edges of an ancient permanent lake.[11] The boundaries of the lake expanded and contracted in a non-cyclical manner in response to gradual climate change over time.[11] The mud at the bottom of the lake near the edges would be exposed during dry periods.[11] This environment was "ideal" for the preservation of dinosaur walking tracks.[11] Coombs estimated that the water was between 1.5 and 2.5 m deep based on the size of the dinosaur swim tracks.[11] The bed preserving the swim tracks almost entirely lacks sedimentary features formed by currents like ripple marks or groove and flute casts.[11] Coombs interprets this absence as implying that the lake had calm water.[11] Coombs also thought that the dinosaur swam out into the water deliberately because the sediment preserving the track was a lake deposit rather than an environment like a floodplain where the dinosaur may have been caught in a flood.[11]

The trackmaker was a large two-legged three-toed animal.[11] Coombs thought the spacing between the toes most closely resembled the ichnogenus Eubrontes, specifically the species E. platypus.[11] Anchisauripus minusculus is another ichnospecies with similarly spaced toes.[11] Coombs referred the swim tracks to Eubrontes, however, because that ichnogenus is the most common among the walking footprints in the area.[11]

The eight print trackway was made by a smaller dinosaur that had a very long step length.[11] Its tracks were on average 94 mm wide, only about half as wide as the average Eubrontes.[11] Its average step length was 104.7 cm.[11] Ichnotaxa with feet of that size tend to have much shorter step lengths.[11] For instance, Grallator formosus has tracks roughly 110 mm wide and a step length of 65.5 cm.[11] Anchisauripus tuberosus has tracks 86 mm wide with a 44 cm step length.[11] Coombs attributed this similarity to two factors:

First, "angulation of the distal metatarsal glinglymi draws the toes together when the foot is strongly flexed".[11] And secondly the animal would have been buoyed by the water and could therefore floated a short distance through the water between footfalls.[11] Coombs referred the smaller swim tracks at the site tentatively to Anchisauripus species.[11]

Megalosaurus and Teratosaurus have both been considered potential candidates for the Eubrontes trackmaker.[11] Large theropods are considered more likely to be responsible for Eubrontes because the ornithopods of the Late Triassic and Early Jurassic were mostly too small to produce them.[11] Also, Eubrontes prints preserve claw marks that are much sharper than the hoof-like or "semi-claw" unguals of ornithopods.[11]

Carnivorous dinosaurs like a coelurosaurs may also have been responsible for Anchisauripus.[11] However, Coombs observed that 90% or more of the Connecticut Valley tracks seem to have been made by theropods, which for ecological reasons couldn't represent their actual percentage in the area.[11] Coombs speculated that it was therefore possible that Eubrontes could have been made by an herbivore.[11] Coombs hypothesized that since there seemed to be swim tracks left by both large an small theropods at the site that swimming was an ability many different kinds of theropod had.[11] Coombs observed that if his interpretations were supported science would have to re-evaluated its understanding about how carnivorous dinosaur pursued prey and how that prey escaped.[11]

Criticism

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The East Berlin Formation dates back to the Hettangian and is part of the Hartford Basin's Newark Supergroup.[12] Many theropod dinosaur footprints preserved in East Berlin Formation rock can be found at Dinosaur State Park near Rocky Hill Connecticut.[12] The rock preserving the tracks is a sandstone consisting mostly of poorly sorted subangular quartz grains.[12] However, mica is also present in particle sizes ranging from sizes comparable to silt to coarse sand grains.[12] Most of it is on the "On the bedding plane surface".[12] The East Berlin Formation is one of several cyclical lacustrine deposits in the Hartford Basin.[12] Most of the theropod tracks were left by large animals and can be attributed to a species in the ichnogenus Eubrontes resembling E. giganteus.[12] There are also tracks left by some small theropods too, however.[12] The rock hosting the footprints seems to be the original surface the dinosaurs walked over because some of the prints are obscured by the rippled-marked sandstone that filled them in.[12] Some of the theropod tracks have atypical shapes.[12] In 1980, Coombs thought that these atypical tracks were made by swimming theropods.[12] These tracks consist of three parallel grooves.[12] The outer grooves narrow near their middles.[12] The middle one tends to be shorter than the outer two and bears a roughly circular impression at its rear.[12] There are three parallel trackways showing this unusual shape, with some of these trackways being left by animals traveling in opposite directions.[12] Trackways left by animals walking at an angle perpendicular to those that left the purported swim tracks are not unusual.[12] One Eubrontes track found isolated from the others exhibits characters halfway between the three-grooved track type Coombs attributed to swimming theropods and a normal walking track with preserved toe pads.[12] One of the unusual tracks was observed partially covered by a normal Eubrontes track.[12] The researchers interpreted the unusual track Coombs thought were left by swimming theropods as being normal theropod tracks that were poorly preserved. What Coombs interpret as drag marks left by the swimming theropods' claws on the lake bottom were actually just impressions left by a theropod's toes while walking normally.[12] The normal Eubrontes tracks that seemed different from the tracks with the parallel grooves wee actually just better-preserved tracks imprinted by later dinosaur activity.[12]

Cameros Basin swim tracks

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In 2007 the researchers regarded the question of whether or not dinosaurs could swim as an unsettled issue in paleontology.[13] However they described new evidence that dinosaur could indeed swim in the form of 12 consecutive theropod tracks from an Early Cretaceous lake deposit in the Cameros Basin of La Rioja, Spain.[13] These theropod tracks had a unique shape that the researchers interpreted as being made by a theropod scratching the lake-bottom with its toes as it swam.[13] The tracks were long and S-shaped.[13] The tracks were located in an ancient upper shoreface with a maximum depth of about 3 meters.[13] The researchers inferred from the track that the theropod was swimming using a "pelvic paddle" technique, which is common in the swimming behavior of modern bipedal animals.[13] In struggling against the current, the theropod used movements similar to those used in walking, but asymmetrical and more forceful.[13] The researchers regarded these tracks as "persuasiv[e] demonstat[ion]" that dinosaurs were able to swim. [13]

Dinosaurs may have had behavior somewhat resembling that of modern mammals because they occupied the same basic ecological niches.[14] The researchers described the question of whether or not dinosaur could swim as "crucial" and "[u]nanswered".[14] The researchers said that although many large mammals were known to be able to swim, there hasn't been any direct evidence that dinosaur could. Elephants evolved the ability to swim in order to cross natural barriers.[14] Bears evolved the ability to swim because they are predators.[14] Dinosaur swimming abilities are significant to research on dinosaur anatomy and ecology because those research programs have become increasingly focused on the nature of dinosaur locomotion.[14] Because data from dinosaur biomechanics and anatomy hasn't demonstrated one way or another, scientists have to rely on the trace fossil record for evidence.[14] Many dinosaur tracks from all around the world are preserved in sedimentary rocks originally deposited by lakes or bodies of flowing water.[14] This may imply that some dinosaurs lived closely associated with water and may have had lifestyles requiring the ability to swim.[14] Although many tracks seem to have been left in watery environments very few have been described as representing actual swimming behavior.[14] In 1980 Coombs reported possible swimming dinosaur tracks, but in 1991 Lockley argued that the tracks represented misleading evidence.[14]

"During the Early Cretaceous, the extensional Cameros Basin formed a vast perrenial lake".[14] This deposit has one of the highest concentrations of dinosaur tracks anywhere in the world.[14] There are more than 10,000 individual dinosaur tracks imprinted in sediments exposed to the surface during the lake's brief lowstands.[14] The dinosaurs were walking across mudflat sediments that later became limestone and through deltaic sediments that later became sandstone.[14] Most of the basin's tracks were left by theropods, some of which have been interpreted as having webbed feet, although the researchers regard this interpretation as unlikely.[14] There are also tracks left by ornithopods and sauropods. [14]

One tracksite is called the La Virgen del Campo tracksite.[14] It is upper Barremian to Aptian in age.[14] and part of the Encisco Group.[14] The researchers describe the tracksite as preserved on a delta that prograded on the southern edge of Lake Cameros.[14] The La Virgen del Campo track site has been productive producing many dinosaur tracks in three zones.[14] The researchers reported a fourth zone that preserves tracks apparently left by swimming theropods.[14] The researchers interpreted the theropods tracks the La Virgen del Campo site as evidence that at least some theropods were competent swimmers.[14]

The La Virgen del Campo site sits on the surface of a 1.5 km wide sheet of sandstone that is roughly 2.5 m thick.[15] The sheet pinches out into bioturbated siltstone at its end.[15] The grain size of the sandstone coarsens upward.[15] There are several different types of facies in the sandstone, including deposits from environments like the open lake, lower shoreface, delta front, and upper shoreface.[15] Upper shoreface facies are found higher up in the sandstone and consist of siltstone in parallel laminations and very fine-grained sandstone interbedded with mudstone.[15] During the deposition of the sandstone, waves left thin ripples in the cross-lamination.[15] Sandstone with fine to medium grain size, plane beds, current ripple forms, and cross-stratificatons scoured the lower shoreface facies.[15] The aforementioned sandstones comprise a meter thick unit deposited in a delta front environment.[15] Beach and upper shoreface facies are positioned at the top of these facies.[15] These deposits consist of a stack of upward fining deposits with symmetrical ripples made by waves during calm weather in very shallow water.[15]

Dinosaur tracks have been found in the upper shoreface facies at La Virgen del Campo.[15] The first three Zones have ripples left by waves with wavelengths of 2.5 cm.[15] Dinosaurs left footprints overtop these ripple marks.[15] Zone 4 has two different sets of ripple marks.[15] The first is a series of round ripples found throughout the Zone.[15] The second set overlies the first and is characterized by vortex ripples with a wavelength of 4.1 cm.[15] This set is half a centimeter thick and found scattered throughout the Zone.[15] These ripples have sharp crests and are a little bit asymmetrical, with a Ripple Symmetry Index of 1.8.[15] They are generally straight but some have subtle curves and branch in a "tuning fork" pattern.[15] The Ripple Index is 7.4, which means the combined flow ripples were "certainly" produced by a current flowing from the northeast.[15] Footprint 3 left scratch marks on the second set of ripples, which means the animal was traveling through the area as the ripples formed.[15]

The trackway is 15 m long and contain 12 tracks consisting of two or three well-imprinted scratch marks.[16] The average stride length of the left leg was 243.3 cm and the average for the right leg was 270.8 cm.[17] The researchers measured from the rear margin of the longest groove in each set of scratch marks.[18] Right-to-left prints had an average oblique pace length of 120.7.[18] Left-to-right prints had an average oblique pace length of 183.1 cm.[18] The average external width of the trackway was 125 cm, and the average internal trackway width was 44 cm.[18]

The researchers designated the three scratch marks composing the tracks as grooves a, b, and c, starting from the interior of the trackway and working out.[18] Each scratch mark was left by the claws and/or toe tips of a theropod dinosaur.[18] There were no foreprints in the track way.[18] The middle groove, groove b, was always the longest scratch mark and the best preserved.[18] Roughly 15 centimeters separate grooves a and b.[18] Most of the scratch marks have a sharp point at the front of the groove.[18] Some of them show a small mound of sediment at the back edge.[18] Both of these features are only found on actual footprints.[18] Further, because the scratch marks were so "delicate" the researchers thought they were unlikely to be ghost traces.[18]

Tracks made by the animal's left leg could be described as two to three parallel scratch marks that sometimes curved slightly into an S-like shape.[18] The points at the front of the scratch marks points in a direction parallel to the direction of the trackway.[18] The sixth left print in the trackway is the only one with a groove c impression.[18] Groove a ranges from 17 to 46 cm long.[18] Groove b ranged from 39.5 to 60 cm long.[18] All but the first of the tracks left by the right leg had three S-shaped parallel grooves.[18] All of them were rotated toward the midline of the trackway at an angle roughly 40 degrees from the track's long axis.[18] Groove a ranges in length from 13.5 to 34.8 cm long.[18] Groove b ranges from 25 to 54.3 cm long.[18] Groove c is always shorter and shallower than those left by the other digits.[18] It ranges in length from 10 to 27 cm. Grooves a and b are always farther apart than b and c.[18]

Prior to their description of the La Virgen del Campo track site there were two kinds of theropod swim tracks reported in the scientific literature.[19] The first of these was reported in 1980 by Coombs from Early Jurassic strata in Connecticut.[19] However, in 2003 Farlow and Galton reinterpreted these tracks as having been left in drying sediment.[19] They observed that similar tracks were found in Australia in sediment that showed physical signs of being in the process of drying.[19] Farlow and Galton also marshalled experimental evidence for their case by observing the tracks a lesser rhea produced while walking on plaster of paris that had nearly dried, and noting their similar features.[19]

The second type of theropod swim track is an ichnogenus called Characichnos tridactylus.[19] However the researchers who studied these tracks had their ability to draw conclusions on dinosaur swimming ability stymied by the tracks' "poor preservational context".[19]

Trackways left by sauropods have been discovered which only preserved foreprints.[19] They have been interpreted as the result of a sauropod traveling through a body of water.[19] However, in 1991 Lockley argued that these were underprints and not left by a swimming animal.[19] Multiple other researchers have reported possible swim tracks left by ornithopods.[19] However researchers in the early 1990s disputed these claims.[19] In 1991 and 1993 respectively Lockley and Bennet responded by arguing that these tracks were either underprints or misidentifications of crocodilian tracks.[19]

In 1983 Currie described a hadrosaur trackway.[20] In 2002 Lee and Huh and in 1994 Lockley and others interpreted manus-only sauropod trackways as being left by dinosaurs walking along the floor of a body of water.[20] In 2004, Thulborn argued against these claims.[21] The authors noted that these sorts of tracks represented walking behavior that happened to be through water rather than true swimming.[21]

The wide variety of length and curved forms of the S-haped tracks are evidence that the trackmaker's body weight was mostly supported by water.[22] The scratch marks were probably left by the animal's claws and the piles of sand at the rear of the claw marks were excavated by the motion of the animal's foot traveling through the sediment.[22] The researchers described these features as common traits of swim track fossils left by reptiles.[22] In 2004 Gierlinski and others reported two isolated scratch marks that had similar traits, and likewise interpreted them as being left by swimming dinosaurs.[22] The Airy wave theory method implies that the medium-sized ripple marks of zone 4 are evidence for water deep enough for the dinosaurian trackmakers to be swimming in.[22] The shallow water waves of the first three zones were probably made by wind.[22] Based on the analytical method, the researchers found that the water in the depositional environment of zone four was at most 3.2 m.[22] They interpreted these results as consistent with the idea that the tracks were left by a partially submerged theropod just barely touching the bottom sediment with its claws while swimming.[22]

In both length and width the dimensions of the trackway are evidence that it was produced by a large animal.[22] This limits the potential trackmaker to being either a turtle, crocodilian or dinosaur since they were the only terrestrial vertebrates large enough during the Early Cretaceous to account for the size of the trackway.[22] Trace fossils left by swimming crocodilians are known from the fossil record.[22] These tracks have prints from both fore- and hindpaws that resemble sets of three to fourth parallel scratch marks in the submerged sediment.[22] They are also bulkier and horter in length than those of the La Virgen del Campo tracks.[22] Crocodilian tracks would also be smaller and more closely spaced.[22] Turtle swim tracks also take the form of scratch marks, but are very different from the La Virgen del Campo tracks.[22] If these tracks had been left by a turtle they would show a wider range of sizes.[22] They would also show only two or three scrape marks and leave evidence of webbing if the turtle rested on the bottom.[22] The grooves in the sediment woulld also only be a few millimeters apart if they were left by turtles.[22] The movement of turtle flippers is also highly synchronized and therefore the trackway should be symmetrical. Tracks left in sand by modern Galapagos tortoises sometimes resemble scratch marks, but in these trackways prints made by the same foot tend to be very close or touch one another.[22] Turtles are therefore rules out as the La Virgen del Campo trackmakers.[22]

Theropod tracks made on fairly solid substrates have some commonalities with the La Virgen del Campo track site.[22] The b claw left the longest and deepest impression in all right tracks and the sixth left track.[22] The tracks have a sharp V-shape because the distances between the a and b grooves are larger than the distance between the b and c grooves.[22] These traits are all seen in walking dinosaur tracks.[22] Therefore, it's very likely that these tracks were made by a theropod.[22] The a, b, and c, grooves were most likely imprinted on the sediment by digits II, III, and IV, respectively.[22]

The theropod making the swim tracks did so using discontinuous movements of the hind limb and left alternating sets of prints in the bottom sediment doing so.[23] Paddle swimming refers to "discontinuous propulsion using the limbs".[23] Tetrapods adapted to land have two different approaches to paddles swimming depending on their gaits.[23] Bipedal animals use pelvic paddle swimming and quadrupeds use pectoral-pelvic paddling motions.[23] The absence of foreprints and the parallel nature of the grooves in the left tracks suggests a theropod using pelvic-paddling motions to swim.[23] This is a bird-like style of swimming.[23] It involves the extension of the legs to propel the animals through the water and then a recovery stroke retracting the limbs into position for the next propulsive stroke.[23] An advanced mesotarsal joint is required for this style of swmming, and is found in dinosaurs.[23] This is more evidence for the authors' hypothesis.[23] The feet must have traveled in a curved path during the swim stroke in order to leave the trackway's S-shaped prints.[23] This implies that the mostion of a theropod dinosaur's legs were similar in both walking and swimming, although the tracks produced by either means of locomotion had very different shapes.[23] This is true of modern animals who use the same style of swimming and supports the researchers' hypothesis.[23]

The right grooves are S-shaped and more promounced than those on the left.[23] The authors interpreted this difference as resulting from a difference in the force exerted by each leg while swimming, with the right leg exerting more force.[23] Difference in stride and pace length of the left and right tracks provide additional support for this idea.[23] The idea that the theropod was swimming in a straight line coinciding with the direction of the trackway is inconsistent with the fact that the right tracks are angled inward about 40 degrees.[23] This would mean that the dinosaur was facing about 20 degrees to the northeast of the trackway direction.[23] The extra force exerted by the right leg was used to maintain this angle. Since the trackway proceeds in a nearly perfectly straight line it's unlikely that the water conditions were still at the time.[23] If the water were still the extra force exerted by the right limb in the course of its swimming stroke would progressively turn the animal leftward.[23] Instead the animal was probably battling a current flowing from the left.[23] Sedimentary structures like the medium-sized ripple marks preserved in the sediment support this contention.[23] The direction their crests face suggest a slow current flowing from the left at an angle roughly 30 degrees to the direction of the trackway.[23]

The authors called the La Virgen del Campo track site the "first long and continuous record" of a swimming non-avian theropod in the trace fossil record.[24] The discovery provides more evidence that dinosaur were able to swim.[24] It also highlight similarities in the swimming behavior of non-avain theropods and modern aquatic birds.[24] Part of this similarity comes from the lack of contact between the forelimb and bottom sediment in both groups of theropod.[24] The tracks also show that dinosaur swimming motions were similar to the motions the legs make during normal walking, but more forceful.[24] The preservation of the tracks was "excellent".[24] It also implies that some non-avian theropods may have lived in palustrine environments.[25] Non-avian theropods may have occupied niches not previously suspected.[26]

Footnotes

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  1. ^ a b c d e "Abstract," Amiot, et al. (2010); page 139.
  2. ^ a b c d e f g h i j k l "Introduction," Amiot, et al. (2010); page 139.
  3. ^ a b c d e f "Materials and Methods," Amiot, et al. (2010); page 140.
  4. ^ a b c d "Results," Amiot, et al. (2010); page 140.
  5. ^ a b c "Discussion," Amiot, et al. (2010); page 140.
  6. ^ "Discussion," Amiot, et al. (2010); pages 140-141.
  7. ^ a b c d e f g h i j k l m n o p q r s t u "Discussion," Amiot, et al. (2010); page 141.
  8. ^ "Abstract," Coombs (1980); page 1,198.
  9. ^ a b c d e f g h i j k l m n o p q r s Coombs (1980); page 1,198.
  10. ^ Coombs (1980); pages 1,198-1,199.
  11. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao Coombs (1980); page 1,198.
  12. ^ a b c d e f g h i j k l m n o p q r s t "Abstract," Rainforth and Howard (2008); page 79.
  13. ^ a b c d e f g h "Abstract," Ezquerra, et al. (2007); page 507.
  14. ^ a b c d e f g h i j k l m n o p q r s t u v w "Introduction," Ezquerra, et al. (2007); page 507.
  15. ^ a b c d e f g h i j k l m n o p q r s t u "Paleoenvironmental Setting," Ezquerra, et al. (2007); page 507.
  16. ^ "Trackway," Ezquerra, et al. (2007); page 507.
  17. ^ "Trackway," Ezquerra, et al. (2007); pages 507-508.
  18. ^ a b c d e f g h i j k l m n o p q r s t u v w x "Trackway," Ezquerra, et al. (2007); page 508.
  19. ^ a b c d e f g h i j k l m "Discussion: Reports of Swimming Dinosaurs," Ezquerra, et al. (2007); page 508.
  20. ^ a b "Discussion: Reports of Swimming Dinosaurs," Ezquerra, et al. (2007); pages 508-509.
  21. ^ a b "Discussion: Reports of Swimming Dinosaurs," Ezquerra, et al. (2007); page 509.
  22. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z "Discussion: Trackway and Trackmaker," Ezquerra, et al. (2007); page 509.
  23. ^ a b c d e f g h i j k l m n o p q r s t u v "Discussion: Swimming Style," Ezquerra, et al. (2007); page 509.
  24. ^ a b c d e f "Conclusions," Ezquerra, et al. (2007); page 509.
  25. ^ "Conclusions," Ezquerra, et al. (2007); pages 509-510.
  26. ^ "Conclusions," Ezquerra, et al. (2007); page 510.

References

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  • Amiot, R.; Buffetaut, E.; Lécuyer, C.; Wang, X.; Boudad, L.; Ding, Z.; Fourel, F.; Hutt, S.; Martineau, F.; Medeiros, A.; Mo, J.; Simon, L.; Suteethorn, V.; Sweetman, S.; Tong, H.; Zhang, F.; and Zhou, Z. (2010). "Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods". Geology. 38 (2): 139–142. doi:10.1130/G30402.1.{{cite journal}}: CS1 maint: multiple names: authors list (link).
  • Coombs, W. P., 1980, Swimming ability of carnivorous dinosaurs: Science, v. 207, p. 1198-1200.
  • Ezquerra, Doublet, Costeur, Galton, Perez-Lorente. 2007. Were non-avian theropod dinosaurs able to swim? Supportive evidence from an Early Cretaceous trackway, Cameros Basin (La Rioja, Spain). Geology 35: 507-510.
  • Rainforth, E.C., and Howard, M. 2008. Swimming theropods? A new investigation of unusual theropod footprints from Dinosaur State Park, Rocky Hill, CT (Newark Supergroup, eastern North America). Geological Society of America Abstracts with Programs 40(2). Page 79.

Further reading

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  • Farlow, J.O. and Galton, P.M., 2003, Dinosaur trackways of Dinosaur State Park, Rocky Hill, Connecticut, in Letourneau, P.M. and Olsen, P.E., eds., The great rift valleys of Pangea in eastern North America, volume 2: New York, Columbia University Press, pp. 248-263.
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